In the Sleep Apnea syndrome a person stops breathing during sleep. Cessation of airflow for more than 10 seconds is called an “apnea”. Apneas lead to decreased blood oxygenation and thus to disruption of sleep. Apneas are traditionally (but confusingly) categorized as either central, where there is no respiratory effort, or obstructive, where there is respiratory effort. With some central apneas, the airway is open, and the subject is merely not attempting to breathe. Conversely, with other central apneas and all obstructive apneas, the airway is closed. The occlusion is usually at the level of the tongue or soft palate. The airway may also be partially obstructed (i.e., narrowed or partially patent). This also leads to decreased ventilation (hypopnea), decreased blood oxygenation and disturbed sleep.
The common form of treatment of these syndromes is the administration of Continuous Positive Airway Pressure (CPAP). The procedure for administering CPAP treatment has been well documented in both the technical and patent literature. An early description can be found in U.S. Pat. No. 4,944,310 (Sullivan). Briefly stated. CPAP treatment acts as a pneumatic splint of the airway by the provision of a positive pressure usually in the range 4–20 cmH2O. The air is supplied to the airway by a motor driven blower whose outlet passes via an air delivery hose to a nose (or nose and/or mouth) mask sealingly engaged to a patient's face. An exhaust port is provided in the delivery tube proximate to the mask. The mask can take the form of a nose and/or face mask or nasal prongs, pillows or cannulae.
Various techniques are known for sensing and detecting abnormal breathing patterns indicative of obstructed breathing. U.S. Pat. No. 5,245,995 (Sullivan et al.), for example, generally describes how snoring and abnormal breathing patterns can be detected by inspiration and expiration pressure measurements made while a subject is sleeping, thereby leading to early indication of preobstructive episodes or other forms of breathing disorder. Particularly, patterns of respiratory parameters are monitored, and CPAP pressure is raised on the detection of pre-defined patterns to provide increased airway pressure to, ideally, subvert the occurrence of the obstructive episodes and the other forms of breathing disorder.
Automatic detection of partial upper airway obstruction and pre-emptive adjustment of nasal CPAP pressure works to prevent frank obstructive apneas in the majority of subjects with obstructive sleep apnea syndrome. However, some subjects with severe disease progress directly from a stable open upper airway to a closed airway apnea with complete airway closure, with little or no intervening period of partial obstruction. Therefore it is useful for an automatically adjusting CPAP system to also respond to a closed airway apnea by an increase in CPAP pressure. However, it is not desirable to increase CPAP pressure in response to open airway apneas, firstly because this leads to an unnecessarily high pressure, and secondly because the high pressure can reflexly cause yet further open airway apneas, leading to a vicious circle of pressure increase.
One method for distinguishing open airway apneas (requiring no increase in pressure) from closed airway apneas (requiring a pressure increase) is disclosed in commonly owned European Publication No. 0 651 971 A1 (corresponding to U.S. Pat. No. 5,704,345). During an apnea, the mask pressure is modulated at 4 Hz with an amplitude of the order of 1 cmH2O, the induced airflow at 4 Hz is measured, and the conductance of the airway is calculated. A high conductance indicates an open airway. This ‘forced oscillation method’ requires the ability to modulate the mask pressure at 4 Hz, which increases the cost of the device. Furthermore, the method does not work in the presence of high leak, and can falsely report that the airway is closed if the subject has a high nasal or intrapulmonary resistance.
The present invention is directed to overcoming or at least ameliorating one or more of the foregoing disadvantages in the prior art.